1. Field of the Invention
The present invention relates to a display device which has an image display element which modulates an illumination light to convert it into a light showing an image, and particularly relates to its illumination optical system.
2. Description of the Prior Art
A display device, which has a liquid crystal display for modulating a given illumination light to convert it into a light showing an image and an eyepiece optical system for guiding the light from the liquid crystal display to an eye so as to provide a virtual image of the image, are used a lot as small image providing means. Such a display device optically conjugates a light source surface emitting the illumination light and the eye of an observer so as to provide a bright image. Moreover, in the case where a color image is provided, the display device generally adopts one of a first method of providing a color filter to individual pixels of the liquid crystal display and modulating white lights as illumination lights collectively and a second method of giving illumination lights with different wavelengths in a manner that time is staggered so as to modulate the respective illumination lights individually.
The first method may have only one light source for emitting a white light, and thus the structure is simple. However, since some pixels of all the pixels of the image display element modulate the lights with respective wavelengths, resolution is low. The second method should have a plurality of light sources and an optical system for guiding lights from the light sources from the same direction to the image display element, and thus the structure is rather complicated. However, since the lights with wavelengths are modulated by all the pixels of the image display element, the resolution is high. In order that the first method provides an image with the same resolution as that in the second method, it is necessary to use a liquid crystal display having a lot of pixels, but since heightening of density of the pixels is limited, enlarging of the liquid crystal display is inevitable. Moreover, when the liquid crystal display is large, a diameter of the optical system for illuminating the display also increases. Therefore, in order to keep the display device small and simultaneously heighten the resolution of a color image to be provided, the second method is preferable.
In the case where the color image is provided by the second method, there arises a problem that irregular brightness and color shading on the light source surface become irregular brightness and color shading on eye surface of an observer. Particularly, the color shading on the light source surface causes lowering of image quality remarkably when the observer shifts his eyes. In order to avoid this, it is necessary to unify color on the light source surface (mix colors). This is generally is realized by arranging a plurality of lights emitting elements for emitting lights with different wavelengths and arranging a diffusing plate in a position which is separated from the light emitting elements. However, with this structure, use efficiency of lights is lowered and miniaturization of the optical system is restricted.
Therefore, it is suggested that optical paths of lights from the plural light emitting elements are superposed by a multilayered film mirror or a reflection type hologram element. They can be used for illuminating a transmission type liquid crystal display and illuminating a reflection type liquid crystal display, but an inexpensive reflection type hologram element which is manufactured by the less steps is more suitable for practical use than an expensive multilayered film mirror which is manufactured by the more steps.
FIG. 17 shows a structure of a display device having a reflection type hologram element. The display device 8 is composed of three light emitting diodes 81R, 81G and 81B for emitting a red (R) light, a green (G) light and a blue (B) light, a hologram element 82, a transmission type liquid crystal display 83 and an eyepiece optical system 85. Lights from the light emitting diodes 81R, 81G and 81B are diffracted to be reflected by the hologram element 82 so as to be guided to the liquid crystal display 83. The hologram element 82 diffracts and reflects the lights from the three light emitting diodes 81R, 81G and 81B to one direction and converts the diffracted and reflected lights into parallel light fluxes. Moreover, the three light emitting diodes 81R, 81G and 81B are arranged on a flat surface which is parallel with an optical path of the diffracted and reflected lights L by means of the hologram element 82 and on an arc, a center of which is one point on the hologram element 82.
FIG. 18 shows a structure of a hologram exposing device to be used for manufacturing the hologram element 82. This device 9 is composed of three laser light sources 91R, 91G and 91B, three mirrors 92a, 92b and 92c, a beam splitter 93, a beam expander 94, two mirrors 95a and 95b and a pin hole plate 96 provided with a pin hole 96a. The laser light sources 91R, 91G and 91B emit laser beams with wavelengths approximately equal with wavelengths of the lights emitted from the light emitting diodes 81R, 81G and 81B.
The mirror 92a reflects a laser beam from the light source 91R. The mirror 92b reflects a laser beam from the light source 91G and allows the laser beam from the mirror 92a to transmit. The mirror 92c allows the laser beam from the light source 91B to transmit and reflects the laser beam from the mirror 92b. The laser beams from the laser light sources 91R, 91G and 91B advance to one direction via the mirrors 92a, 92b and 92c. 
The beam splitter 93 branches the laser beam from the mirror 92c into a transmitted light and a reflected light, and the beam expander 94 expandes a light flux diameter of the laser beam transmitted through the beam splitter 93. The mirror 95a reflects the laser beam reflected by the beam splitter 93, and the mirror 95b further reflects the laser beam from the mirror 95a so as to allow it to cross the laser beam transmitted through the beam expander 94. The pin hole plate 96 allows the laser beam from the mirror 95b to pass through the pin hole 96a so as to convert it into a divergent pencil of lights.
A substrate 82a coated with a hologram sensitizer, namely, an original object of the hologram element 82 is arranged in a position where the laser beam from the beam expander 94 and the laser beam from the pin hole plate 96 cross, and is hologram-exposed. At this time, the substrate 82a is arranged so that an angle made by the laser beam from the beam expander 94 transmitted through the substrate 82a and a normal of the substrate 82a becomes equal with an angle made by the diffracted and reflected lights L at the time of use shown in FIG. 17 and a normal of the hologram element 82.
In addition, the pin hole plate 96 is movable, and the pin hole 96a can be in three positions with respect to the substrate 82a which has the same positional relationship with the three light emitting diodes 81R, 81G and 81B with respect to the hologram element 82. FIGS. 19(a) through 19(c) show states that the hologram exposure is carried out. As shown in FIGS. 19(a) through 19(c), the hologram exposure is carried out three times in a manner that the position of the pin hole 96a is changed, and in the respective hologram exposures, one of the three laser beam sources 91R, 91G and 91B emits a laser beam with wavelength approximately equal with that of the light emitting diode corresponding to the position of the pin hole 96a. 
The hologram element 82 obtained in such a manner has the function for diffracting and reflecting lights from the light emitting diodes 81R, 81G and 81B in one direction and mixing the colors, and the function for converting the diffracted and reflected lights into parallel light fluxes suitable for illuminating the liquid crystal display 83. The light emitting diodes 81R, 81G and 81B can be arranged near the hologram element 82, and the display device 8 can be miniaturized easily.
However, in the display device 8, since the three light emitting diodes 81R, 81G and 81B are arranged on the flat surface parallel with the diffracted and reflected lights L by means of the hologram element 82, the hologram exposing device 9 becomes complicated and large. The reason for this will be explained with reference to FIGS. 18 and 19.
At the time of the three-time hologram exposure, it is necessary to make a direction of the substrate 82a with respect to the laser beam from the beam expander 94 constant. Meanwhile, it is necessary to change an angle of the laser beam from the pin hole 96a with respect to the laser beam from the beam expander 94 at each time of the hologram exposure. Namely, it is integrant that the pin hole plate 96 is movable, and at least the mirror 95b of the mirrors 95a and 95b for guiding the laser beams to the pin hole plate 96 should be also movable. For this reason, a mechanism for holding the pin hole plate 96 and the mirror 95b so that they are movable to a direction of an arrow is unexpendable.
Furthermore, the mirror 95b is moved simply together with the pin hole plate 96, and also it is necessary to change the angle formed by the mirror 95a and the pin hole plate 96 at the time of each hologram exposure, and a mechanism for this is further necessary. When these mechanisms are provided, the hologram exposing device 9 becomes complicated and large.
In addition, in the display device 8, incident angles of the lights from the light emitting diodes 81R, 81G and 81B with respect to the hologram element 82 are different from one another, and a difference in the incident angle of the lights from the two light emitting diodes 81R and 81B positioned on both ends is large. Generally, as a difference between the incident angle and the diffracting and reflecting angle is smaller, the diffraction efficiency of the reflection type hologram element is higher, and as the incident angle is smaller, the diffraction efficiency is higher. Moreover, generally a radiation angle of a small light emitting diode is 60° in full width at half maximum, namely, wide. For this reason, in order to utilize a radiated light effectively, it is more preferable that an area of the hologram element where the light enters is larger.
Taking this into consideration, in the display device 8, the hologram element 82 is arranged so as to face between the light emitting diodes 81R, 81G, 81B and the the liquid crystal display 83. However, even with this structure, it is inevitable that the the diffraction efficiencies of the lights from the light emitting diodes 81R and 81B differ, and the diffraction efficiency of the light from the light emitting diode 81B which is the farthest from the liquid crystal display 83 is easily lowered. An output of the light emitting diode 81B is heightened relatively, so that an image with suitable color can be provided. However, the lowering of the diffraction efficiency is not preferable from the viewpoint of the effective use of lights.
Further, when the light emitting diodes 81R, 81G and 81B are arranged on the flat surface parallel with the diffracted and reflected lights L of the hologram element 82, there arises problem that a structural dimension becomes large.